Security Tools

Is your toolbox getting pretty full from my last two articles on monitoring tools and troubleshooting tools? Well, make some room. This article covers six security tools that every administrator ought to know.

From the author of

From the author of

Monitoring Tools

This article presents six tools that should become part of your security toolkit.
nmap is a port scanner that has become the de facto standard for system
and network administrators. Nessus is a security scanner that has replaced
the older SATAN as the tool of choice for catching known security problems.
iptables is the interface to the next generation of Linux packet filtering
and IP masquerading. Xinetd and tcp wrappers provide two methods
of controlling access to a specific host. They can be used separately or in
conjunction with one another. OPIE provides a password-obscuring mechanism
to allow fairly secure logins without encrypting the session between two hosts.

nmap

nmap is a tool for scanning a machine (or machines) for security problems and
is exercised from the network. nmap is often run from the command line, but a
GTK+-based front end (nmapfe) is also available. nmap is written and
maintained by
Fyodor. nmapfe
was originally written by
Zach Smith and is now
maintained by Fyodor.

nmap and related tools are something of a mixed blessing, and thus have a
murky reputation. While these tools are incredibly useful for a system or
network administrator, they can also be used to the advantage of system
crackers. Some people would prefer that tools like nmap weren't made
publicly available. I tend to side with the other part of the community. If
tools like this weren't available to the good guys, it would give the bad
guys an incredible advantage because they certainly won't give up their
tools.

nmap is an external security scanner, or a port scanner. It works by sending
IP packets to the host(s) it is checking and by watching for what kind of
response (if any) it receives.

nmap presents a summary of the responses it receives to provide a security
overview of the target host(s).This overview can show

A list of open ports

Owners of remote processes on open ports

RPC services matched against the port on which they are provided

Information about TCP sequence numbers

Remote operating systems

Getting and Installing nmap

Binaries and source of nmap are available from
http://www.nmap.org. The
binaries are as easy to install as you would expect. Building nmap and nmapfe is
easy as well.

After putting the source tarball into /usr/local/source, you just follow the
regular three step process:

Several different kinds of scans available through nmap are described in
Table 1.

Table 1 Scans Available Through nmap

Scan Type

Switch

Description

TCP connect() scan

-sT

The most basic form of scanning. This opens a connection to every potentially
interesting port on the target machine. Any user can use this kind of
scan. It is easily detected, with many connection messages showing up
in the logs of the target machine.

TCP SYN scan

-sS

The "half-open" scan. This scan sends a TCP SYN packet as though
it is trying to open the connection. If it receives a SYN-ACK response,
it sends an immediate RST to shut down the connection. Because this scan
doesn't open the connection, it is less likely to be logged. Only
users with root privilege can send TCP SYN scans.

Stealth FIN

-sF

This scan attempts to pass through packet filters by sending a TCP FIN
packet.

Christmas Tree

-sX

This scan attempts to pass through packet filters by sending a packet
with FIN, URG, and PUSH flags set.

Null

-sN

This scan attempts to pass through packet filters by sending a packet
without any flags turned on.

Ping

-sP

This limits the scan to only conducting a ping sweep to look for connected
systems. It does not do port scans.

UDP scan

-sU

This sends 0 byte UDP packets to each port on the target machine(s).

ACK scan

-sA

This scan is used to help check packet filters. An ACK packet with random
acknowledgment and sequence numbers is sent. If nothing is returned, the
port is marked as filtered.

Window scan

-sW

This scan is similar to the ACK scan, but depends on anomalies in the
TCP window size handling of some OSes.

RPC scan

-sR

This scan checks all open ports found by other scan types and sends RPC
NULL commands to see if they are RPC ports. If they are RPC ports, this
scan attempts to determine what program and version number they serve.

FTP bounce scan

-b <ftp relay host>

This scan relives a historical foible of FTP servers. Older FTP servers
were able to serve bounced FTP sessions; that is, they connected to another
host to deliver data to you. By providing a relay host in the format username:password@server:port,
you can use this FTP bounce (mis)feature to scan ports that might otherwise
be protected.

In addition to the types of scans that nmap can run, a number
of options modify its behavior. These options include timing, target
identification, output, and others. Some of the more useful options are shown in
Table 2.

Table 2 nmap Options

Option

Explanation

-P0

Tells nmap not to ping hosts before scanning. (This is used to scan hosts
that sit behind packet filters that don't allow ICMP traffic.)

-PT<PORT>

Uses TCP to look for hosts on the target network. ACK packets are sent,
and nmap looks for RST packets to be returned. An optional PORT number
can be given. It tells nmap which port to attempt its connections against.

-PS

Causes nmap to use SYN packets instead of ACK packets when checking for
hosts on the target network (root users only).

-PI

Uses ICMP only when looking for hosts on the target network.

-PB

Uses both ICMP and TCP ACK to sweep the target network for hosts. This
is the default behavior.

-O

Causes nmap to attempt remote host identification based on the way the
target system handles TCP packets containing certain types of errors.

-I

Causes nmap to use an identd scan. This causes the owner of each server
process to be shown as well.

-f

Causes nmap to fragment its scanning packets, making it more difficult
to block the scan with packet filters.

-v

Puts nmap into verbose mode, causing it to display much more information
about what it's doing.

-oN <logfile>

Writes output into a human readable logfile.

-oM <logfile>

Writes output into a machine-parsable logfile.

--resume <logfile&;

Resumes an incomplete scan from a logfile.

-iL <logfile>

Causes nmap to read input from a logfile instead of really scanning a
host.

-g <portnumber>

Allows you to define the port nmap uses as its source port.

-p <port range>

Allows you to define the range of ports nmap will scan. If no range is
given, nmap will scan all the ports listed in its own services file. A
range of ports can be given in the following format: -p 20-30,79,6000-6010.

nmap allows you to list target IP addresses in one of four
styles. For single hosts, you can write the IP address or hostname. For
networks, you can write the number in slash notation using the CIDR-style
network mask (for example, 192.168.1.0/24). The most flexible form of target
listing allows you to wildcard portions of the address (or list them as ranges).
This allows you to look at specific hosts within a group of networks (for
instance, if you know that all your routers use the first IP address of their
Class C address, you can scan your internal routers with a target such as
192.168.*.1). Finally, you are also able to provide a list of hosts (in any of
the previous styles).